We consider a relativistic plasma of charged particles that is not necessarily in equilibrium, but is in the regime where a quasi-particle description is valid.
We use an effective phase-space description in terms of distribution functions that are not assumed thermal or isotropic.
A convenient parametrization for the distribution function of a spheriodially anisotropic system was introduced in [1]}, [2]}.
The idea is to introduce one parameter that effectively stretches or squeezes the spherically symmetric isotropic distribution in one direction.
This parametrization is interesting in the context of heavy ion collisions where one expects the parton distribution that is produced by the collision to be squeezed along the beam axis.
Dispersion relations are obtained from the poles of the dressed propagators of the partonic quasi-particles.
One finds that the spectrum of collective excitations is much richer than the spectrum of a system in thermal equilibrium.
It is especially interesting that there are imaginary modes, which are associated with instabilities and could play an important role in the thermalization of the plasma [3]}, [4]}, [5]}, [6]}, [7]}, [8]}.
Anisotropic plasmas have been studied intensively in the context of quark-gluon plasmas, which thermalize earlier than expected, for reasons that are as yet unclear.
Collective modes have been calculated for both gluons [1]}, [2]}, [11]} and quarks [12]}.
An ellipsoidal generalization was developed in [13]}, [14]}.
These distributions have been used to study various properties of quark-gluon plasma (QGP) including
transport coefficients [15]}, [16]}, [17]}, [18]},
heavy quark bound states [19]}, [20]}, [21]}, [22]},
and photon and dilepton production [12]}, [24]}, [25]}, [26]}, [27]}.
